About 3 weeks ago the tyre pressure monitor pinged for my left front tyre only. I checked it, it was 2-3lbs below the others. I blew it up, re-set it and carried on. I've done quite a bit of driving since,  around 2k miles, and it pinged again today, same tyre.

 

Realising it is now an issue I went to a local tyre place. They took it off sprayed it with magic spray that should show air bubbles but they found nothing. No charge, 'I couldn't find anything so I couldn't fix anything '.

 

I thought it would be a slow puncture but perhaps not.

 

Any ideas?

I've seen it before when it was the beading seal. Nothing seen in a water bath but a re- seat sorted the problem.

Magic spray of any sort is fairly unlikely to find a porous wheel casting. But I'd always have any recently fettled tyre re-seated (like john999boy says) if it still drops pressure I'd get a new tyre/valve (BUT keeping old tyre if 3mm+) and see if the  warning still pings, if it does then it's fairly likely that the rim is iffy, but any good refurbers (new word?) 'automatically' fix most problems such as bead seat damage (from tyre monkeys - apologies to none simian tyre professionals) as they re-lacquer the rim bed, and bead seat, well pretty much everything whilst the rim is spinning in the air.

 

If the tyre/rim hasn't been recently interfered with, then a leak is likely to be just a slightly punctured tyre that's under load, not many magic sprays are calibrated to detect that....

Thanks folks 👍

if replace valves i recommending smarter ones -

Direct TPMS: https://www.briskoda.net/forums/topic/434028-tire-pressure-monitoring-system/?tab=comments#comment-4893182

 

Kids got a paddling pool?  Take it off and submerge it.

 

2-3 lbs in a couple of weeks is a very slow leak

The magic spray is a good dose of fairy liquid or similar in water in a spray bottle. It foams when air bubbles through it. If you have time, take the wheel off lay it levelled on its side and spray the fairy onto the rim / rubber and leave it. after some time you will see white foam in one spot. mark it with gaffa tape, refit and go back to the tyre place.